1. Field of the Invention
The present invention relates to a holder for a drive piston of a setting tool.
2. Description of the Prior Art
European Publication EP-O 346275 B1 discloses an explosive powder charge-operated setting tool including a piston guide and a drive piston displaceable in the piston guide. The piston guide has radial openings facing the drive piston, and spring-biased braking balls engaging the drive piston. The spring, which applies a biasing force to the braking balls is formed as a ring spring for applying a radially acting, with respect to the piston, biasing force to the braking balls. The ring spring is provided on its inner profile with a bearing surface acting on the braking ball. The bearing surface is inclined to the piston at an acute angle that opens in a direction opposite a setting direction. In the ignition ready position of the drive piston, the braking balls engage the outer surface of the drive piston under the action of the ring spring. When the drive piston moves in the setting direction, it entrains the braking balls therewith. The braking balls expand the ring spring, which results in the bearing surface transmitting the radial biasing force to the braking balls in the direction toward the drive piston. The braking balls are pressed radially against the piston body by the ring spring. Even with a small displacement of the drive piston in a direction opposite the setting direction, the braking effect can be substantially reduced or eliminated, as the braking balls displace in the same direction as the drive piston, unloading the ring spring. After being unloaded, the spring washer does not press any more the braking balls against the piston body. Further, a possibility still remains that the drive piston would be displaced, before ignition or firing of the setting tool, in the setting direction as a result of, e.g., the setting tool being pressed too hard against a constructional component. The displacement in the return direction is effected due to cooperation of the spring washer with the braking balls.
U.S. Pat. No. 4,162,033 discloses a setting tool with a braking element that continuously applies a braking force to the drive piston.
An object of the present invention is to provide a piston holder having a simplified design and which would reliably retain the drive piston in its ignition-ready position in the absence of ignition.
This and other objects of the present invention, which will become apparent herein after, are achieved by providing a piston holder for a drive piston of a setting tool and including a support or a carrier for two expansion legs for frictionally receiving the drive piston therebetween, which carrier is fixedly secured in the setting tool.
The two expansion legs extend at an acute angle to each other and are resiliently deflectable relative to each other, forming together a resilient clamping device. The expansion legs overlap the drive piston in such a way that the inner edges of the two expansion legs apply pressure to approximately diametrically opposite circumferential sections of the drive piston. Advantageously, the two expansion legs lie in a plane that forms with the axial or drive-out direction of the drive piston an acute angle opening toward the front end of the setting tool.
When the drive piston moves in the setting, drive-out direction, i.e., toward the front end of the setting tool, the friction force, which is applied by the expansion legs to the drive piston or to its body, increases. With increase of the displacement path of the drive piston, a holding or braking force acting on the drive piston also increases due to the increase of the wedge action between the expansion legs and the drive piston. However, when the drive piston-displacing force exceeds a predetermined value, the expansion legs elastically expand, releasing the drive piston. In this way, the expansion legs act as a quasi overload protection means. Upon its release, the drive piston just slides through the guide channel and drives an object, e.g., a fastening element in, e.g., a constructional component. In this way, practically, there is obtained a speed-dependent friction coefficient that provides for reduction of friction at a high relative speed between the drive piston and the expansion legs. A total braking or a complete stop in this way is prevented.
The expansion legs do not hinder return movement of the drive piston when it returns to its initial, ignition-ready position after the completion of a drive-in or setting process, as the friction between the drive piston and the expansion legs is still very small.
Due to the prestress of the expansion legs relative to each other, a small pressure is constantly applied to the drive piston or its body. Thereby, the drive piston is reliably held in its ignition-ready position in the absence of ignition of the setting tool. If an undesirable displacement of the drive piston takes place as a result, e.g., of the setting tool being pressed too hard against a constructional component, the expansion legs would become loaded in the drive piston drive-out direction, whereby a restoring force is generated that provides for displacement of the drive piston into its initial position. The piston holder simultaneously provides for reduction of undesirable rebounds of the drive piston.
Generally, both expansion legs can be formed as separate parts securable on their carrier. However, according to an advantageous embodiment of the present invention, the expansion legs form legs of a V-shaped spring, forming parts of a one-piece element. Forming the expansion legs as the legs of a V-shaped spring facilitates their mounting and reduces manufacturing costs of the piston holder. The spring can be wound with its other end about a bolt secured in the carrier. This provides for preloading of the spring in the direction opposite the setting direction. Because of such preloading of the V-shaped spring, the expansion legs cannot rotate in the direction opposite the setting direction. For preventing the rotation of the expansion legs in the direction opposite the setting direction, also a suitable stop can be provided.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to is construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments when read with reference to the accompanying drawings.